Force a Blender operator to always start with default values
Add 'SKIP_SAVE' to the blender property option set.

When defining properties for the operator, any with 'SKIP_SAVE' in their property definition options set, will (like the name suggests) not be saved, and revert to their default value the next time the operator is invoked.

    scale_factor = bpy.props.FloatProperty(name="Scale Factor", 
            default=1,
            options={'SKIP_SAVE'})